1. Field of the Invention
The present invention relates to the field of bearingless couplings between the blade and rotor hub in a helicopter, and in particular relates to the flexures in such bearingless couplings.
2. Description of the Prior Art
Bearingless couplings between helicopter blades and rotor hub are well known to the art and numerous versions have been designed and used in an attempt to provide a rugged and flexible coupling between the blade and hub.
Derschmidt et al., "Rotor Without Flapping Hinges and Without Lead-Lag Hinges", U.S. Pat. No. 4,201,515 shows a blade-to-hub coupling based upon a plurality of coupling arms formed within a generally cylindrical envelope by exploitation of a multiple rod design. Ditlinger, "Flexible Coupling", U.S. Pat. No. 3,893,788 also shows a multiple rod design for connecting a helicopter blade to a hub wherein the rods are disposed in a generally cylindrical pattern.
Blade-to-hub couplings having a small cross section to the direction of rotation of the rotor system are also well known. For example, Ormiston et al., "Hingeless Helicopter Rotor with Improved Stability", U.S. Pat. No. 3,999,886 uses a radially extending arm from the hub to provide flexure about the radial axis in the rotor plane as well as for lead-lag deflections and out-of-plane flapping. A similar flexure is described by Roman et al., "Composite Hingeless Rotor Hub for Rotary Wing Aircraft", U.S. Pat. No. 4,111,605 where flat straight straps, generally extending in the radial direction, are paired and used for blade flexure in much the same way as the hingeless coupling of Ormiston. A similar composite flexure is described by Bourquardez et al., "Rotor Construction", U.S. Pat. No. 3,669,566.
Many other designs for using a flex beam coupling between blade and blade and blade and hub are known to the art. Each of these designs are particularly characterized by using elastomeric or fiber composites in complex coupling structures which have reasonably large size and bulk in directions normal to the rotor plane. Examples of such flex beam couplings are shown by Derschmidt, "Helicopter Rotor Head", U.S. Pat. No. 4,182,597; Kisovec, "Rotary Head Assembly for Rotary Wing Aircraft", U.S. Pat. No. 3,880,551; Gorndt et al., "Rotor Blade Retention System", U.S. Pat. No. 3,862,812; Rybicki, "Cross Beam Rotor", U.S. Pat. No. 4,093,400; and Baskin, "Rotary Head Assembly for Rotary Wing Aircraft", U.S. Pat. No. 3,874,815.
What is needed is a bearingless coupling which is rugged, simple in design, inexpensive to manufacture, and which provides a minimum of drag to the rotary motion of the blade and hub system, but which at the same time provides for controlled flexure of lead-lag angles, flapping, and pitch of the blades. These and other objects of the present invention are achieved by the invention as briefly described below.